Convert WiFi RSSI to Signal Bars (Formula and Code Snippet)

I went way down the rabbit hole trying to convert RSSI (in dBm) to a useful signal strength meter, like you’d see on your cell phone.

I read many articles, but no one really had an algorithm.  So….here is an algorithm written in Javascript.

The Goods

//This assumes you have a way to obtain rssi... in my application,
//  this is the rssi for the web server's connection (IoT device).

function rssiToBars( rssi , maxbars ){
  var ret=rssi+80;        //Useful range is -80 (bad) to -30 (good).
                          //+50 converts this to a useful range of 0 (bad) to 50 (good)

  ret=(ret< 0)?0:rssi;   //Clamping
  ret=(ret>50)?50:rssi;

  ret=parseInt(ret*maxbars/50);    //taking a percent (rssi/50) of maxbars

  // If you are using a language with integer data types, make sure you multiply
  // first, then divide.

  return(ret);
}

function rssiMeter( bars , maxbars ){
  return("<PRE>[" + "#".reapeat(bars) + "-".repeat(maxbars-bars) + "]</PRE>");

  //This is a simple example, but you could stack a couple of small PNG or 
  //render a table where the cells are bars.
}

const maxbars=8
var rssi=-55;   //some magical way to get the rssi
var bars=rssiToBars(rssi,maxbars);
var meter=rssiMeter( bars , maxbars);

// rssi  == -55
// bars  ==  4
// meter ==  <PRE>[####---]</PRE>

How It Works

Received Signal Strength Indicator (RSSI) is the relative signal strength of a transmitter, from the receiver’s perspective.

RSSI is measured in dBm, or Decibel-milliWatts.  Basically, this is milliWatts as perceived by the receiver, but represented on a logarithmic scale.

Because power drops inversely with the square of the distance, a logarithmic scale more or less represents this as a linear scale, rather than exponential.

For example, if you have signal strength of -30 (very good) and you walk 50 feet away with no obstacles between you and the router, you might drop to -40 dBm.  If you walk another 50 feet, you would expect the signal to drop to -50 dBm.  This a super-simple example, and there are a lot of other factors involved, but you get the basic idea.

The algorithm works like this:

1. Convert rssi to a useful scale.  We do this by adding 80.  If we start with a range of -30 (good) to -80 (bad) and add 30, we get 0 (good) to -50 (bad).  Further if we add 50, we get a positive integer that is now reversed:  -50+50==0 (bad) and 0+50==50 (good).
2. Next, we clamp the values to prevent unexpected results, ensuring we now have an integer in the range of 0..50.
3. Dividing this number by 50 yields a percentage (0..1)
4. Multiplying the percentage by the max number of bars, gives you the number of bars.  For example, .5 * 8  (medium signal strength * 8 bars) == 4.
5. If using a typed language such as c++, be sure to multiply first, THEN divide:  (rssi+50)*maxbars/50.  If not, the CPU will perform int math, and bars will be zero most of the time.

Rendering can be done however you want.  One slick trick is to use CSS and a small table, where each cell is a bar, and CSS dictates the cell color.  Your code sets the class of each cell to “bar” or “nobar”.

<STYLE>
.bar , .nobar {
  height: 10px;
  width: 10px;
  border: 1px solid gray;
  border-collapse:collaps;
  font-size: 1px; 
  /* Each cell has a non-breaking space in order to force it to render */
  /* Setting font-size to 1px ensures that the height of the cell will be honored */
}

.bar{
  background-color: #0000FF //Blue
}

.nobar{
  background-color: #000077 //Dark Blue
}
</STYLE>
<SCRIPT>
  function rssiMeter( bars , maxbars ){
    var meter="<TABLE STYLE='border-collapse:collapse;'><TR>";

    for(var i=0 ; i<maxbars ; i++){
      var class=(i<bars)?"bar":"nobar";
      meter+="<TD CLASS="+class+">&nbsp;";  
      //NO I DO NOT CLOSE MY TD TAGS.  SUE ME
    }

    meter+="</TR></TABLE>";

    return(meter);
  }

  // you can attach it to a DIV like this:
  const maxbars=8;        // define constants at the top of your script
  const divname="meter"; 

  function updateMeter( rssi ){
    var bars = rssiToBars(rssi , maxbars); //maxbars is a global const
    var meter= rssiMeter( bars , maxbars); 
    document.getElementById(divname).innerHTML=meter;   //divname is a global const
  }

  // be sure there is <DIV ID=meter>&nbsp;</DIV>
  // somewhere in your document

  //Now, whenever you want to update your meter:
  var rssi = somenumber;
  updateMeter( rssi );

</SCRIPT>

updateMeter(-55);

would produce:

Enjoy!

Excel – Number of Sundays – A better Answer

I looked up how to count the number of Sundays between two dates…. the so-called answer shocked me.  There are a couple of popular answers, but they are horribly inefficient.

Let me show you a better way…..

Update 4/22/2025

Due to a severe copy and paste error, the formulas were completely wrong.  8-(

These have now been corrected.

Solution

Let’s say we have the following cells:

We will follow the Excel convention, where Sunday = 1, to find the number of “1” days (cell B3) between 1/1/2025 (cell B1) and 5/1/2025 (cell B2).

Between the two dates:

=INT((B2-(B1+B3-WEEKDAY(B1)+7))/7)+1

Correctly returns 17

Between and including the two dates:

=INT((B2-(B1+B3-WEEKDAY(B1)+7))/7)+1+IF(WEEKDAY(B1)=B3,1,0)

Correctly returns 17.  If we were to change B1 to 1/5 (a Sunday), the result would be 16 without the extra IF term.

For a more detailed explanation…

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